1. Field of the Invention
The present invention relates to a tyre for two-wheeled vehicles and to a process for manufacturing such a tyre.
In particular, the present invention pertains to a tyre for two-wheeled vehicles provided with a reinforcing structure obtained by applying strip-like elements along the circumferential extension of a toroidal support. Each of said strip-like elements comprising longitudinal reinforcing thread elements disposed parallel to each other and at least partly coated with at least one layer of elastomeric material.
2. Description of the Related Art
A tyre for vehicle wheels usually comprises a carcass structure essentially made up of one or more carcass plies shaped into a substantially toroidal configuration and having their axially opposite lateral edges in engagement with respective annular reinforcing structures incorporating circular metal inserts, usually referred to as “bead cores”. Each annular reinforcing structure is embedded in a so-called “bead” for anchoring the latter to a corresponding mounting rim.
Applied to the carcass structure, at a radially external position, is a belt structure comprising one or more belt layers in the form of a closed ring, essentially made up of textile or metallic cords suitably oriented relative to each other and to the cords belonging to the adjacent carcass plies.
Further applied to the belt structure, at a radially external position, is a tread band usually consisting of a strip of elastomeric material of suitable thickness. It is to be pointed out, to the aims of the present specification, that by the term “elastomeric material” it is intended a compound comprising at least one elastomeric polymer and at least one reinforcing filler. Preferably, this compound further comprises additives such as cross-linking agents and/or plasticizers, for example. Due to the presence of the cross-linking agents, this material can be cross-linked through heating, so as to form the final article of manufacture.
A pair of sidewalls is applied onto the opposite sides of the tyre, each of them coating a side portion of the tyre included between a so-called shoulder region, located close to the corresponding lateral edge of the tread band, and the corresponding bead.
Recently, particular attention has been paid to the research of production methods enabling the manufacture of intermediate semifinished products intended for tyre building to be eliminated or at least limited. For instance, in the European patent EP09288680 in the name of the same Applicant, it is disclosed a method of manufacturing tyres in which the carcass ply or plies, as well as each of the belt layers, is obtained by laying a plurality of strip-like elements one after the other in a circumferentially approached relationship, on a toroidal support such shaped as to match the inner conformation of the tyre to be obtained.
In document WO01/38077 in the name of the same Applicant it is disclosed another method and an apparatus for manufacturing a belt layer for vehicle tyres through laying of strip-like elements cut from a continuous strip-like element. In detail, the cut strip-like elements are applied in mutually approached relationship without leaving any space between one strip-like element and the one subsequently laid, along the circumferential extension of a toroidal support. The aforesaid application allows at least one reinforcing layer to be obtained which has a continuous circumferential extension around a geometric rotation axis of the toroidal support. During this application, between the strip-like element and the toroidal support a relative angular rotation is determined around a correction axis that is substantially radial to the geometric rotation axis of the toroidal support to avoid overlapping of the ends of the strip-like elements in the vicinity of the tyre shoulder. As compared with tyres for four-wheeled vehicles, tyres for two-wheeled vehicles are required to offer a quite peculiar performance involving many structural differences. One of the most important differences results from the fact that when a two-wheeled vehicle is running on a bend, it must tilt laterally to a great extent as compared to its position when running on a straight stretch, thus forming with the perpendicular to the ground an angle (referred to as camber angle) that can reach 45°, or even 65° under hard handling conditions. Therefore, when the two-wheeled vehicle negotiates a bend, the contact area of the tyre progressively moves from the central region of the tread towards the axially outermost region in the direction of the curve centre. For this reason, tyres for two-wheeled vehicles are distinguishable by their marked transverse curvature. This transverse curvature is usually defined by the particular value of the ratio of the distance between the radially external point of the tread and the line passing through the laterally opposite extremities of the tread itself measured on the equatorial plane of the tyre, to the distance measured along the tyre chord between said extremities.
In tyres for two-wheeled vehicles, the value of the curvature ratio is generally at least as high as 0.15 and is usually in the order of about 0.3 in the case of rear tyres and even higher, until about 0.45, in the case of front tyres, against a value usually in the order of about 0.05 in tyres for four-wheeled vehicles.
In addition, in tyres for two-wheeled vehicles the belt structure can be made up of one or more continuous cords wound into axially approached coils substantially parallel to the circumferential extension direction of the tyre (the so-called “zero-degree belt”).
Alternatively, the belt structure can consist of two radially superposed layers, each consisting of elastomeric material reinforced with cords disposed parallel to each other, said layers being such arranged that the cords of the first belt layer are oriented obliquely to the equatorial plane of the tyre, while the cords of the second layer have an oblique orientation as well, but symmetrically crossed relative to the cords of the first layer (the so-called “crossed belt”).
The orientation angle of the cords of each layer is very important for obtaining some features in the tyre. For instance, to a low value of this angle (tending to) 0°), both on the shoulder and on the crown, a high stability in the straight stretch corresponds, but a poor lateral thrust.
On the contrary, to high values of said angle, both on the shoulder and on the crown, a high ride comfort corresponds, but a more reduced stability in cornering.